This invention relates to plastic film handling and processing equipment, and is more specifically directed to an improved bow spreader bar for stretching the web of film transversely as it passes between a supply roll and a pick-up roll so as to eliminate creases and wrinkles in the plastic film. The invention is more particularly concerned with improvements to the bow spreader that permits rapid and accurate change in the arc or bow and also in the pitch angle of the arcuate member relative to the film web, and in making it possible to adjust the pitch angle and arc remotely, which facilitate set up and adjustment where the film path is in an elevated or difficult location. The concepts of the present invention can reduce maintenance issues, and can eliminate problems of catching or tearing of the film.
Polyethylene film or films of other plastic materials are typically produced as elongated webs of film and are stored as rolls of plastic film for further processing. The film can be fed from a supply roll, through one or more processing stages, and then taken up on another roll. This processing can include printing, die cutting, punching, seam forming etc. Typically, the web of film has a stress imposed on it in the longitudinal direction of the web, i.e., the “machine” direction, with relatively less stress imposed in the transverse direction. As a result, the plastic film can become creased or wrinkled prior to being rewound on the take-up roll. These creases and similar flaws can interfere with subsequent printing, punching, or cutting steps, and can decrease the value of the end product. Consequently, it has become the practice in plastic film handling to impose a transverse stress on the film web by means of a bow or arm.
The bow spreader is typically positioned adjacent to the plane of the web in advance of the take-up roll. The bow spreader is arced or bowed so as to press into the web, and can be arranged so as to penetrate the web plane at an angle perpendicular to the web plane, although the pitch of the bow spreader can be set to a lower angle. Commonly, bow spreaders are formed as a bowed rod carrying a row of rollers end to end. There can be a rubber covering over the rollers. The intention is that the rollers and covering will turn or rotate about the rod. In order to turn the existing bow rollers, the bow has to press heavily into web and create significant web tension. Alternatively, the rollers may be power-driven to match the line speed. In that case the powered bow spreader bar requires a speed sensor and a feedback circuit so that the rollers match the line speed exactly. This increases the cost and complexity of the device. There is also wear on the rollers from use as they rub against one another. The bowing of the spreader creates gaps between rollers on the convex side, which can catch on the film web and on the covering. All of these factors mean that the roller-based spreader bars are expensive and complex and require considerable maintenance.
An improvement to the roller-based spreader bar is found in a static bar that may be formed of a metal extrusion with an arcuate cap or contact surface that is treated with a low-friction surface treatment, and which may employ a low-friction semi-rigid cover fitted onto it. One example of this is found in U.S. Pat. No. 6,113,026 to Pottorff. This improved bow spreader bar has a film-facing surface formed of a low-friction material, and can take the form of an extruded arcuate member adapted to be positioned transversely in contact with the film web. This can have a rounded contact surface over which the film slips past. As shown in that U.S. Patent, the extrusion may be in the form having a generally T-shaped profile with a top that forms the curved or arcuate contact surface and a step that may be formed of one or two tubular components, which may serve as a conduit for a cooling fluid. The surface treatment may take the form of a lubrication hard-coating impregnated into the extrusion, which favorably is an aluminum alloy. A suitable semi-rigid plastic resin cover may be fitted onto the top of the extruded bow spreader bar, which may have an incorporated low friction agent.
A need remains to simplify the adjustment of bowing needed for the spreader bar for each given run of plastic film, and also to adjust the pitch angle of the bow along the transverse axis, and to enable the bow spreader bar to be set and adjusted remotely to avoid having to send a worker into a potentially difficult or dangerous zone within the film processing equipment.
A need also exists for a bow spreader bar that can be easily and quickly adjusted during set up or during a run, if necessary, without requiring the worker to put his or her hands on the equipment.